Arduino

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Biking in place through Google StreetView. Buzzword warning: VR environment for stationary cycling.

More usefully described, this project uses a head-mounted-display that has sensors for detection of orientation so that the user can be placed into a Google StreetView scene and can look around freely. For movement through the map, an exercise bike used to estimate forward movement using a simple reed switch that counts the number of revolutions of the wheel. Pragmatically explained, it means that I don't have to sit in the darkness or stare at my garage door while I'm huffing away.

Hopefully once this is done, I'll be able to spend a few nights a week pedalling away downstairs and work my way across the US or Australia over the course of the year.
Top 40 Arduino Projects of the Web. How To Smell Pollutants. This Instructable explains how to use a gas sensor with your Arduino.

This lets your Arduino smell (and hence you program responses to) overall gas levels for a variety of nasties, including ethanol, methane, formaldehyde, and a bunch of other volatile organic compounds. My cost to make this actual device was under $100, including the full Arduino kit.
Arduino All-in-One Getting Started Guide. An all-in-one tutorial to getting started with the Arduino open-source electronics prototyping platform.

This guide is meant for the beginner but should be also be useful to you if you already tinker with electronics but want to get started with the Arduino. I'll cover: